1. Field of the Invention
The present invention relates to a humidification system for a fuel cell which utilizes a solid high polymer membrane as an electrolyte membrane, and which is employed in a fuel cell vehicle, and in particular, to a fuel cell humidification system with a water-permeable-type humidifier and to a fuel cell humidification system using a water-permeable-type humidifier as a heat exchanger.
2. Description of the Related Art
Conventionally, a fuel cell provided with a solid polymer membrane is known. In such a type of fuel cell, the generation of electricity is carried out by electrons, generated by a chemical reaction between externally supplied oxygen and hydrogen, and which flow in the solid polymer membrane. In order to conduct efficient electrical generation, the solid polymer membrane should be highly conductive, to make that resistance to the movement of the electrons generated by the reaction should be sufficiently low.
The reaction in the fuel cell generates water, and thus exhaust gas exhausted from the fuel cell after the reaction contains a considerable amount of moisture. Accordingly, as disclosed in Japanese Unexamined Patent Application. First Publication No. Hei 6-132038, a humidifier for humidifying a reaction gas used for the reaction in the fuel cell has been proposed by applying the exhaust gas exhausted after the reaction to the humidification gas.
Although the conventional fuel cell humidification system is advantageous in efficiently utilizing the exhaust gas, the problem arises in this humidification system that, since the exhaust gas contains a large amount of moisture, the moisture content within the fuel cell becomes rich as the reaction gas is humidified by the exhaust gas. As a result, the moisture condenses into dew in the spaces between the solid high polymer membranes in the fuel cell, thereby disadvantageously clogging the gas passage, and decreasing the generation performance.
Further, with the fuel cell, in order to conduct the efficient electricity generation, the recommended driving temperature of the fuel cell should be set to a predetermined temperature depending on the output. When the temperature is above this predetermined temperature, the solid polymer membrane may be damaged. When the temperature is below that predetermined temperature, the moisture collects between the solid high polymer membranes due to the dew condensation and the catalytic inactivation, thereby clogging the membranes, and decreasing the generation performance.
Conventionally, to set the inlet gas temperature to the driving temperature, a heat exchanger through which cooling water flows cools the gas at the inlet, thereby maintaining the inside of the fuel cell below the suitable temperature.
However, a conventional system using the heat exchanger for reducing the inlet gas temperature requires a comparatively large heat exchanger, radiator fan, and cooling water pump, has increased electric power consumption, and occupies a large space. Further, these various parts disadvantageously increase the weight of the vehicle.
Furthermore, to make the solid polymer membrane highly conductive to reduce resistance to the movement of the electrons generated by the reaction, the fuel cell requires humidification of the inlet gas.
It is therefore an object of the present invention to provide a fuel cell humidification system which can humidify the reaction gas depending on a dew point command.
It is another object of the present invention to provide a fuel cell humidification system which can effectively utilize a humidifier for humidifying the inlet gas to the fuel cell, and which can maintain the inlet gas for operating the fuel cell at a suitable temperature.
In the first aspect of the present invention, the humidification system for humidifying a fuel cell comprises: a water-permeable-type humidifier for humidifying a reaction gas used in a reaction with moisture in an exhaust gas exhausted after the reaction; and an adjuster for adjusting an amount of humidification to a humidification amount required by the fuel cell.
The first aspect of the present invention increases the ratio of the reaction gas which flows through the humidifier to all of the reaction gas supplied to the fuel cell, or increases the ratio of the exhaust gas which flows through the humidifier to all of the exhaust gas exhausted from the fuel cell. By means of this, the amount of humidification can be increased depending on the amount of humidification required by the fuel cell. Further, the first aspect of the present invention deceases the ratio of the reaction gas which flows through the humidifier to all of the reaction gas supplied to the fuel cell, or decreases the ratio of the exhaust gas which flows through the humidifier to all of the exhaust gas exhausted from the fuel cell. By means of this, the amount of humidification can be decreased depending on the amount of humidification required by the fuel cell. Thus, the appropriate humidification conditions of the fuel cell can be maintained, and the fuel cell can be used most efficiently.
In the second aspect of the present invention, the humidification system further comprises: a reaction gas supply passage from the humidifier to the fuel cell; and a reaction gas bypass passage, connected to the reaction gas supply passage, for allowing the reaction gas to bypass the humidifier and for controlling an amount of gas flow.
According to the second aspect of the present invention, as the amount of the reaction gas flowing through the reaction gas bypass passage is increased, the amount of the humidified gas in all of the reaction gas supplied to the fuel cell can be relatively decreased. As the amount of the reaction gas flowing through the reaction gas bypass passage is decreased, the amount of the humidified gas in all of the reaction gas supplied to the fuel cell can be relatively increased. Thus, the amount of the reaction gas flowing through the reaction gas bypass passage is controlled depending on the amount of humidification required by the fuel cell, and the fuel cell can be used most efficiently.
In a third aspect of the present invention, the humidification system further comprises: an exhaust gas discharge passage from the fuel cell through the humidifier; and a reaction gas bypass passage, connected to the exhaust gas discharge passage, for allowing the exhaust gas to bypass the humidifier and for controlling the amount of gas flow.
According to the third aspect of the present invention, as the amount of the exhaust gas flowing through the exhaust gas bypass passage is increased, the amount of the exhaust gas, which flows through the humidifier to humidify the reaction gas in all of the exhaust gas exhausted from the fuel cell is relatively decreased. As the amount of the exhaust gas flowing through the exhaust gas bypass passage is decreased, the amount of the exhaust gas which flows through the humidifier to humidify the reaction gas, in all of the exhaust gas exhausted from the fuel cell is relatively increased. Thus, the amount of the exhaust gas flowing through the exhaust gas bypass passage is controlled depending on the amount of humidification required by the fuel cell, and the fuel cell can be used most efficiently.
In a fourth aspect of the present invention, a humidification system for humidifying a fuel cell comprises: a humidifier for extracting moisture from gas exhausted from the fuel cell and for supplying the moisture to an inlet gas for operating the fuel cell; and an adjuster for adjusting a pressure or flow of the exhausted or inlet gas passing through the humidifier to control the temperature of an entering gas.
According to the fourth aspect of the present invention, the inlet gas supplied to the fuel cell and the gas exhausted from the fuel cell are supplied to the humidifier which conducts the heat exchange between the exhausted gas and the inlet gas. Therefore, as the pressure of the inlet gas to the fuel cell is increased, or as the amount of flow of the inlet gas is decreased, the temperature of the inlet gas is increased. As the pressure of the inlet gas to the fuel cell is decreased, or as the amount of flow of the inlet gas is increased, the temperature of the inlet gas is decreased. Thus, the humidification system does not require a heat exchanger, and effectively utilizes the humidifier to suitably adjust the temperature of the inlet gas supplied to the fuel cell.
Further, as compared with the conventional system with a heat exchanger, the space required for the system can be decreased, and the weight of the vehicle is not significantly increased.